How would building a wall along the US-Mexico border affect animals?

The border between the US and Mexico is 2,000 miles long. It passes through deserts, mountains, rivers, streams, forests, wetlands, marshes, and swamps. Many of these are biologically diverse. Some species live nowhere else on Earth. Fences harm wildlife, disrupt animal behaviors, and block migration routes. Those are just a few of the problems walls can cause.